Inkjet cartridges are now well known in the art and generally comprise a body containing an ink supply and having electrically conductive interconnect pads thereon and a printhead for ejecting ink through numerous nozzles in a printhead. In thermally activated inkjet cartridges, each cartridge has heater circuits and resistors which are energised via electrical signals sent through the interconnect pads on the cartridge. Each inkjet printer can have a plurality, often four, of cartridges each one having a different colour ink supply for example black, magenta, cyan and yellow, removably mounted in a printer carriage which scans backwards and forwards across a print medium, for example paper, in successive swaths. When the printer carriage correctly positions one of the cartridges over a given location on the print medium, a jet of ink is ejected from a nozzle to provide a pixel of ink at a precisely defined location. The mosaic of pixels thus created provides a desired composite image.
Inkjet cartridges are increasingly becoming more sophisticated and complex in their construction and longer lifetimes are also required of cartridges, particularly those for use with printers having an off-carriage ink reservoir which replenishes the cartridge's ink supply. This has lead to greater sophistication in the so-called "servicing" of cartridges by a printer. It is normal for printers to have a service station at which various functions are performed on the cartridges while they are mounted in the printer carriage such as wiping, spitting and capping, see for example U.S. Pat. No. 5,585,826. Wiping comprises moving a wiper of a specified material across the printhead of a cartridge to remove paper dust, ink spray and the like from the nozzle plate of the printhead. Spitting, ejecting ink into a spittoon in the service station, is performed to prevent ink in nozzles which have not been fired for some time from drying and crusting.
Cartridges are capped by precisely moving the printer carriage, and often the cap too, within the service station, so that the cap mates with the printhead of the cartridge and forms a seal around the nozzle plate. Capping prevents ink on the printhead and in the nozzles from drying by providing the correct atmosphere around these components and thus reduces the risk of crusting and ink plug formation in the nozzles. Also the cartridge can often be primed while in the capped position by the application of a vacuum through the cap. It can thus be seen that an effective seal must be formed between the printhead and the cap to facilitate these functions. Caps are usually formed of a resiliently deformable material such as rubber and in use are ideally pressed against a printhead of a cartridge with a substantially constant force, the capping force, chosen so as to achieve an effective seal with the printhead. While this is relatively easily achieved for a printer carriage having a single cartridge, ensuring that all the cartridges of a printer carriage having a plurality of cartridges are effectively capped is considerably harder. A number of arrangements are known, see for example U.S. Pat. No. 5,563,638, in which a plurality of caps are mounted on a spring-loaded gimbal mechanism in an attempt to achieve a constant capping force between each of the caps and its respective printhead. However, manufacturing tolerances unavoidably cause there to be differences between each cap and cartridge pair and the remaining pairs. These differences can often result in different capping forces for each cap and cartridge pair so that some pairs receive insufficient capping force and others receive too great a capping force which may damage the printhead. In an attempt to alleviate these problems an improved cap has been designed as disclosed in the commonly assigned, issued U.S. Pat. No. 5,448,270 by Osbourne, which is incorporated herein by reference. Although the cap described in '270 is effective in achieving a substantially constant low capping force over a greater deflection for each cap and cartridge pair than prior caps, it has been found that there is nevertheless still undesirable and unpredictable interaction between different pairs of caps and cartridges which affects their accurate mating.